Oil shale is the colloquial term for a wide variety of laminated sedimentary rocks containing organic matter that can be released predominantly only by destructive distillation. While some removal of organic matter by solvents is possible, the amount so removed is quite small and is not feasible on an economical basis. This characteristic permits a clear distinction from tar sands which are rock or sand formations actually impregnated with oil.
Oil shales generally contain over one-third mineral matter. The organic portion, a mixture of complex chemical compounds, has been termed "kerogen". Kerogen is simply a generic name for the organic material found in such circumstances, but it is not a definite material since kerogen compositions differ when derived from differing shales.
While oil shales have been utilized for centuries as a source of fuel, such uses have generally been small, and the great potential for the huge deposits in various locations around the world remains to be unlocked on a feasible commercial scale.
Shale oil is a dark, viscous organic liquid obtained by pyrolyzing oil shale. Refining of the shale oil is similar to the handling of crude petroleum as far as the basic refining steps and end use products are concerned. Shale oil, of course, is not "crude oil". Destructive pyrolysis of crushed shale yields shale oil, but under the pyrolysis conditions commonly employed, a disproportionation of carbon and hydrogen structures equivalent to internal hydrogenation is believed to occur. A large percentage of this heavy kerogen converts to a liquid, some to light gases, and the rest remains as a carbon-rich residue on the inorganic matrix. Shale oil in some respects may be considered as intermediate in composition between petroleum and coal tar, comparing for example the C:H atomic ratio of about 6:7 for petroleum, about 7:9 for shale oil, and about 10:16 for coal carbonization products.
The Tosco process of retorting oil shales employs a cocurrent flow of hot ceramic balls and oil shale in a rotating drum means. The oil shale takes up heat from the balls, and the oil vapors produced are drawn off into a collection system, leaving a spent shale admixed with the balls. The spent shale is transferred to a furnace where residue-carbon is burned off to provide reheating of the balls. The main advantages of the Tosco system are the relatively high throughput rates achieved in proportion to the size of equipment, and the production of high-BTU off-gas since there is no dilution thereof by combustion products. However, one serious disadvantage of the Tosco process has been just how to separate the ceramic balls from the spent shale.